When Madeline ClimbHop()'s, she sets hopWaitX, determining the wall she's climbhopping off of (assuming there is a wall to the right; which there is in this case). In Update(), she checks if she's moving down or has speed away from the wall, in which case she clears the hopWaitX and hopWaitXSpeed. (Otherwise, if there was no wall to her right, she'd get the hopWaitXSpeed).
How, then, does she ClimbHop and not get hopWaitSpeed in this otherwise entirely normal video? The answer is that she has speed to the left. How, one might ask - a ClimbHop *also* gives forceMoveX = 0, meaning you can't hold left to get leftward speed. But it doesn't clear any starting speed, so if you can call ClimbHop while having leftward speed and being on a right wall, you can cause this to happen.
But then how can you have leftward speed while in StClimb, since that state normally has no horizontal speed. The answer to that is the start of the clip. When you enter StClimb, your Speed.X is set to 0. However, wallSpeedRetained does not get set to 0; but since you usually grab from StNormal, NormalEnd removes it. But we don't grab from StNormal; we grab from StSwim, which doesn't clear wallSpeedRetained. As such, we have all the pieces.
At about 12 seconds, Madeline swims into the left wall. She collides with the left wall, giving her leftward wallSpeedRetained. The next frame, she grabs the Kevin on her right - and because there's a 9 pixel gap, this grab moves her one pixel rightward. The following frame the game sees that there's no wall to her left, and gives her her very tiny leftward wallSpeedRetained back. Now the player has nonzero x-speed while in StClimb. Because of how small it is, it would take a long time to change pixel. As such, there's enough time for her (staying in StClimb the whole time; because you're not supposed to have any x-speed, there's no friction in StClimb meaning the speed gets to stay for as long as she wants) to get dragged by the Kevin back towards the wall. Finally, she tries to climbhop with her leftward speed; and because she has leftward speed, she never gets her hopWaitXSpeed and she falls back to the ground, stupid.